The seed value is used when calculating random numbers in a computer. This value affects the randomness of the shatter when Maya calculates the cracks. If you change the seed value, the way the object is cracked changes.
By using the same seed value you will always get the same set of random numbers. So, if you don't like the way Maya shattered your surface, you can change the seed value to get it to shatter in a different way. When you find a shatter that you like, you can keep that seed value in and it will always shatter the same way.
This will be the case with any other node in Maya that has a seed value attribute.